1. Technical Field
This invention relates to an organic electroluminescent (EL) device using a polymer, and more particularly, to a molecule dispersed polymer type organic EL device using a vinyl polymer.
2. Background Art
Multilayer structure organic EL devices using low molecular weight compounds obtained by a vacuum evaporation process (as described in, for example, Appl. Phys. Lett., 51, p 913 (1987)) are now on the verge of commercialization as a result of consecutive fabrication of devices on a practically acceptable level. A number of structures have been proposed for the low molecular weight compounds. It was proposed, for example, in JP-A 8-12600 that a blue light emitting device having high efficiency is established using a 9,10-diphenylanthracene derivative with a high solid quantum yield as a host material in a light emitting layer, with the proposal becoming of great interest.
On the other hand, active efforts have been made on the development of organic EL devices using high molecular weight compounds or polymers. The use of polymers is expected to overcome the thermal stability problem which arises with the low molecular weight compounds obtained by an evaporation process and to simplify the manufacturing process because of possible coating. Polymeric organic EL devices can be generally divided into π-conjugated type using conjugated polymers and molecule dispersed type having dyes dispersed in unconjugated polymers.
With regard to the π-conjugated type device, since its disclosure (as described in JP-A 10-92576, for example), the development of polymeric organic EL devices fabricated by coating polymers has been accelerated. Lamination with an excellent hole-injecting polymer such as polyethylenedioxythiophene/polystyrene sulfonic acid (PEDOT/PSS) has provided a device having characteristics comparable to the device relying on the evaporation process.
However, the use of π-conjugated polymers has the problem that the spreading of π-conjugated system renders it awkward to make a material emitting blue light of a good color purity. No material exhibiting a color purity and lifetime both within the practically acceptable region is available at present.
The molecule dispersed type was announced earlier than the π-conjugated type (see, for example, Polymer, Vol. 24, 748–754, 1983). By mixing in a host polymer a dopant commonly used in the evaporation type, blue light of good color purity is readily obtained. However, as compared with the low molecular weight compound evaporation type and the π-conjugated type, the molecule dispersed type suffers from a serious delay with respect to properties, especially lifetime.
The cause is the difficulty of polymer synthesis, and only a few compounds have been evaluated. The polymers which have been used in the molecule dispersed type include polyvinyl carbazole (PVK) and only a few reported so far (see, for example, Kido et al., Polymer Preprints, Japan, 45, No. 3, 446 (1996), Kido et al., Chem. Lett., 161 (1996)). Vinyl polymers having an anthracene structure have also been reported, but with extremely poor properties (see, for example, Kido et al., Polymer Preprints, Japan, 50, No. 4, 661 (2001)). The vinyl polymers reported therein include a homopolymer obtained from 9-(4-vinylphenyl)anthracene in the form of a 9- or 10-substituted anthracene structure and copolymers obtained from 9-(4-vinylphenyl)anthracene and N-vinylcarbazole.